1. Field of the Invention
The present invention relates to a speaker apparatus for converting an electrical signal into an acoustic signal, and more specifically, to a structure for improving the sound quality.
2. Description of the Related Art
Conventionally, acoustic reproduction is performed by a speaker system 1 having a basic structure as shown in FIG. 7. In the speaker system 1, one or a plurality of speaker units 2 are accommodated in an enclosure 3. The speaker unit 2, which in many cases assumes a generally conical cross-section, has a vibration plate 4 called “cone.” The speaker unit 2 is also equipped with a magnetic circuit 5, which has a main magnet 6, a center pole 7, and a plate 8. In a magnetic gap between the center pole 7 and the plate 8 is concentrated magnetic flux generated by the main magnet 6 in high density. A voice coil 9 whose tip is joined to the base portion of the vibration plate 4 is suspended in the magnetic gap.
When the voice coil 9 is energized, driving force acts on the voice coil 9 in the magnetic gap and the vibration plate 4 is thereby displaced, whereby sound waves are emitted from the vibration plate 4 to the neighboring air. Each speaker unit 2 is accommodated in the enclosure 3 to prevent back-side sound waves (opposite in phase to front-side sound waves) from going around the speaker unit 2 to the front side. Each speaker unit 2 has a frame 10 for use in fixing of the magnetic circuit 5 and for vibratably supporting the vibration plate 4. The frame 10 is fixed to the enclosure 3.
Having a structure called “external magnet type,” the magnetic circuit 5 is suitable for a case where a ferrite magnet is used as the main magnet 6. However, the external magnet type magnetic circuit 5 leaks a large amount of flux to the outside. Where the speaker system 1 is used together with a cathode-ray tune (CRT) for acoustic reproduction as part of an audio-visual apparatus such as a TV receiver, a video player or acoustic reproduction for a personal computer or a game machine, there is fear that a color purity error or a distortion may occur and lower the image quality. Countermeasures for decreasing the leakage magnetic flux include attaching a cancellation magnet 11 to the rear side of the magnetic circuit 5 and, in addition, covering the magnetic circuit 5 with a shield cover 12.
The electromagnetic driving force acting on the voice coil 9 is transmitted to the neighboring air from the vibration plate 4. The vibration plate 4 applies pressure to the neighboring air and receives reactive force therefrom. The reactive force that the vibration plate 4 receives is transmitted to the magnetic circuit 5 through electromagnetic interaction between the voice coil 9 and the magnetic circuit 5 and then transmitted from the magnetic circuit 5 to the enclosure 3 via the frame 10. Therefore, in the speaker system 1, when sound is outputted from the vibration plate 4 by driving each speaker unit 2 electrically, the speaker unit 2 itself vibrates and this vibration is transmitted to the enclosure 3. Sound is also emitted from the surfaces of the enclosure 3. Being opposite in phase to the sound emitted from the vibration plate 4, this sound interferes with the sound emitted from the vibration plate 4. As such, this sound is a factor of deteriorating the quality of sound emitted from the speaker system 1 as a whole. Further, because of reaction to the movement of the vibration plate 4 for emitting sound, the center pole 7 side of the magnetic circuit 4 tends to vibrate. Therefore, the efficiency of energy transmission from the vibration plate 4 to the air is low, which influences the transient characteristic of sound and, in terms of the sound quality, lowers a sense of speed to be given to a listener.
Japanese Unexamined Patent Publications JP-A 5-153680 (1993), JP-A 11-146471 (1999), etc. disclose a technique in which in the enclosure each speaker unit is not fixed to the front side of the frame of the speaker unit but to the rear side of the magnetic circuit. By fixing the magnetic circuit to a grounding surface to make vibration hard to be transmitted to the magnetic circuit and to be transmitted from the frame to the enclosure, it is expected that the degree of sound emission from the enclosure will be lowered and the deterioration of sound quality will be decreased.
To strongly support the magnetic circuit portion while accommodating each speaker unit in the enclosure as in the above prior art technique, it is necessary to, for example, make the enclosure of a dividable type and assemble the enclosure after completion of support of each speaker unit. This results in problems that the number of assembling steps of the speaker apparatus increases and the structure of the enclosure becomes complex. There may be cases where the enclosure cannot be divided as exemplified by a speaker that is attached to a vehicle door as the enclosure.